Abstract

Regimes of diagenesis. — Secondary sandstone porosity is formed and preserved, changed and destroyed in almost any conceivable diagenetic environment. The system of diagenetic regimes that Choquette and Pray (1970) formulated for the study of porosity in carbonate rocks is equally appropriate for investigation of secondary porosity in sandstones and has been adopted in this paper. Eodiagenesis is defined as the regime at or near the surface of sedimentation where the chemistry of the interstitial water is mainly controlled by the surface environment prior to effective burial. (Effective burial is here considered as burial under strata that seal the sandstone from a predominant influence of surface agents on the chemistry of the interstitial water). Mesodiagenesis is the subsurface regime during effective burial. Telodiagenesis represents the regime at or near the surface after effective burial. The amounts of secondary porosity originating in each diagenetic regime differ greatly (Figure 33).

Eodiagenesis produced a minor portion of secondary porosity, mainly as a result of dissolution of sedimentary and eogenetic constituents, although fracturing and shrinkage may be subordinate contributors. In some instances eogenetic processes created considerable secondary porosity, for example, in calcareous quartz sandstones of Pleistocene and Holocene age from the Trucial Coast (unpublished petrographic study by Schmidt).

Mesodiagenesis created by far the largest amount of secondary porosity, and most of it can be attributed to leaching of sedimentary, eogenetic, and mesogenetic carbonate constituents (Hayes, 1979; Schmidt, 1976). Fractures form an important, although subordinate, portion of mesogenetic sandstone porosity.Mesogenetic shrinkage porosity plays a minor